By custom and convention, high-density polyethylene pipe is O.D.-controlled. That is to say, the capacity of high-density polyethylene pipe is specified by its outer diameter. However, ductile iron, P.V.C. and steel pipe is I.D.-controlled; their capacities are specified by their inner diameters. Heretofore, there was no convenient means for reliably joining high-density polyethylene pipe to these other pipes.
To illustrate the difficulty, for a given capacity requiring a 12 inch ductile iron pipe, the 12 inch ductile iron pipe would have a 12 inch nominal O.D. and a 12.58 inch I.D. but a comparable high-density polyethylene pipe would have a nominal O.D. of 16 inches and an I.D. of 12.444 inches. In order for high-density polyethylene pipe to be used to join with ductile iron, P.V.C. or steel pipe, some form of adapter must be employed.